1. Field of the Invention
The present invention relates to a circuit for generating a direct current (DC) control differential base voltage. More particularly, the present invention relates to a DC controlled differential base voltage generating circuit applicable to a gain control of an RGB video amplifier for controlling a contrast by a voltage generating circuit in which an input control signal has a voltage within a set range so that it can easily interface with a computer controlled alignment system and so that it has a high input impedance.
2. Description of the Prior Art
In general, all kinds of video signal processing devices including a television receiver, has a contrast control section at an output terminal of an RGB video amplifier. In such video amplifier for controlling a contrast, a gain variation thereof causes a contrast of a video signal to change. In order to control a gain of the video amplifier for controlling a contrast, a conventional method controls a voltage fed back from an output to an input by adjusting a variable resistance so that the gain of the video amplifier is changed.
FIG. 1 shows a conventional differential base voltage generating circuit for controlling a gain of a video amplifier in order to control a contrast by using a variable resistor.
Referring to FIG. 1, the conventional differential base voltage generating circuit includes a differential amplifier having transistors Q5, Q6, Q7 and Q8 and resistors R6, R7 and R8, which receives a difference voltage between both bases of the sixth and seventh transistors Q6 and Q7, that is, a differential base voltage .increment.V.
A potential divider 5, constituted by a first resistor R1 and a second resistor R2 is arranged between a driving voltage Vcc and a pair of diode-connected transistors Q1 and Q2, where Q2 is grounded, and the connection of the R1 and R2. A pair of diode-connected transistors Q1 and Q2, for maintaining a constant voltage from the ground, is connected in series with the base of a transistor Q6. Therefore, a constant voltage is always generated between first resistor R1 and second resistor R2 of the divider 5 and is applied to a base of the third transistor Q3 so that the third transistor Q3 is turned on. In consequence, a voltage across an emitter resistor R4 of an emitter-follower transistor Q3 is applied to a base of an emitter-follower transistor Q4. Accordingly, transistor Q4 is turned-on.
A voltage across an emitter resistor R5 of transistor Q4 is applied to a base of transistor Q6 of differential amplifier 1. Therefore, first, second, third and fourth transistors Q1, Q2, Q3, and Q4, first resistor R1, second resistor R2, third resistor R3, fourth resistor R4, and fifth resistor R5 constitute a reference voltage generating section 3 and; serve to supply a reference voltage to base of the sixth transistor Q6.
An emitter of a feed back transistor Q8 is connected to a base of the seventh transistor Q7, a base thereof is connected to a collector of the seventh transistor Q7, and a collector thereof is connected to a driving voltage Vcc through a third resistor R3. A seventh resistor R7 is connected between the base of the seventh transistor Q7 and a ground. A variable resistor R.sub.V is connected to a collector of the seventh transistor Q7 through a ninth resistor R9.
A differential base voltage .increment.V of differential amplifier 1 constructed as above is calculated by means of an expression (1) which will be described as follows. EQU .increment.V=V.sub.BE6 -V.sub.BE7 =V.sub.T 1nI.sub.6 /I.sub.s -V.sub.T 1nI.sub.7 /I.sub.s EQU =V.sub.T 1nI.sub.6 /I.sub.7 ( 1)
As described in expression (1), the differential base voltage .increment.V is determined by currents I.sub.6 and I.sub.7 flowing through the sixth and seventh transistors Q6 and Q7, respectively. The current I.sub.7 is the sum of a current I.sub.8 flowing through an eighth resistor R8 and a current I.sub.9 flowing through a ninth resistor R9. Therefore, the conventional differential base voltage generating circuit controls the variable resistor R.sub.V to adjust the value of I.sub.9 so that the differential base voltage .increment.V can be generated.
Currently, in high quality video systems, the video control is performed by a computer. However, since the conventional differential base voltage generating circuit controls the variable resistor R.sub.V to adjust the value of I.sub.9 so that the differential base voltage .increment.V can be generated, the conventional differential base voltage generating circuit has a low input impedance and cannot easily interface with a computer controlled alignment system.
U.S. Pat. No. 5140181(issued to Hiroshi Yoshino on Aug. 18, 1992) discloses one example of a direct current differential base voltage generating circuit. U.S. Pat. No. 5,140,181 relates to a reference voltage source circuit used in a bipolar monolithic integrated circuit such as an audio circuit. According to the reference voltage source circuit, the emitters of the transistors constituting a difference amplifier are biased by a constant current source. A current mirror circuit is connected to the collectors of the transistors constituting the differential amplifier. A reference potential circuit for generating a reference potential is connected to the base of one of the transistors constituting the differential amplifier, and an output current from the current mirror circuit is fed back to the base of the other transistor. Therefore, the reference potential generated from the reference potential circuit is applied to the base of the other transistor. The reference potential applied to the base of the other transistor is applied to the base of a transistor of Darlington-connected transistors which receives an input signal, and the reference potential serves as a bias power source voltage. U.S. Pat. No. 5,140,181 has a high current gain and low distortion, but does not have a high impedance.
U.S. Pat. No. 4,658,205, (issued to Kazuyoshi Yamada on Apr. 14, 1987) discloses another example of a direct current differential base voltage generating circuit. U.S. Pat. No. 4,658,205 relates to a reference voltage generating circuit wherein a temperature characteristic of an output voltage is set as desired. The reference voltage generating circuit is constituted by a source of a constant current, a power supply having high and low voltage terminals, an emitter follower circuit connected across the high and low voltage terminals and the output terminal of the source of constant current and the other ends connected to the input terminal of the emitter follower circuit, a third resistor with one end connected to the high voltage terminal and a diode with its anode electrode connected to the other side of the third resistor and its cathode electrode connected to the output terminal of the source of constant current. U.S. Pat. No. 4,658,205 has a temperature characteristic not influenced by the temperature characteristic of the driving voltage of a constant current source, but does not have a high impedance which can easily interface with a computer controlled alignment system.